The present invention relates to organic electroluminescence (EL) devices.
Organic EL devices have received much attention for use as a display device or a thin luminescence device. A typical organic EL device includes a transparent electrode (anode), which is made of indium tin oxide (ITO) and formed on a glass substrate, an organic EL layer, which has a luminescence layer formed on the transparent electrode, and a cathode, which is formed on the organic EL layer. The luminescence layer generates light that is emitted from the glass substrate.
Drive voltage is applied between the anode and the cathode so that current flows through the organic EL layer between the anode and the cathode to emit light from the electroluminescence layer. The intensity of the light emitted from an organic EL device (luminescent brightness) is proportional to the amount of current flowing between the anode and cathode. Further, the resistance of the material forming the transparent electrode is greater than that of the material forming a metal electrode. Accordingly, the electric resistance and electric current differs between a location near an electrode terminal and a location far from an electrode terminal. The difference between the electric currents causes the brightness to differ at different positions in the organic EL device.
Japanese Laid-Open Patent Publication No. 2003-115393 describes a first prior art example in which luminescent brightness differences are suppressed. To reduce the electric resistance of a transparent electrode in the first prior art example, a transparent electrode is used. The transparent electrode has a multilayer structure including a transparent conductive layer and a light-transmissive metal film. The metal film decreases the electric resistance of the transparent electrode and reduces the differences in the luminescent brightness. However, the metal film decreases light transmission in the transparent electrode. This decreases the brightness of the organic EL device.
FIG. 6 shows an organic EL device that suppresses brightness unevenness without decreasing the brightness of the organic EL device. The organic EL device 41 includes a transparent electrode 43, which is formed on a glass substrate 42, and a metal auxiliary electrode 44, which is formed on a peripheral portion of the transparent electrode 43 and connected to an anode. An insulation film 45 is formed on the auxiliary electrode 44. An organic EL layer 46 covers the transparent electrode 43 and the insulation film 45. A cathode 47 is formed on the organic EL layer 46. A protective film 48 is formed on the outer side of the cathode 47 to protect the organic EL layer 46 from oxygen and moisture.
Japanese Laid-Open Patent Publication No. 2003-123990 describes a second prior art example of an organic EL device 54, as shown in FIGS. 7, 8A, and 8B. The organic EL device 54 includes a transparent substrate 50, a transparent anode layer 51, an organic EL layer 52, a cathode layer 53, an insulation layer 55, and an auxiliary electrode layer 56. The transparent anode layer 51 includes an anode electrode terminal 51a. A cathode electrode terminal 53a is defined on the opposite side of the anode electrode terminal 51a. The auxiliary electrode layer 56 includes a contact region 57 that has three sides contacting the transparent anode layer 51 (FIG. 8A) or two sides contacting the transparent anode layer 51 (FIG. 8B)
In the organic EL device 41 of FIG. 6, the organic EL layer 46 is thin. Thus, the organic EL layer 46, which is arranged between the auxiliary electrode 44 and the cathode 47, is apt to being cut by edges of the auxiliary electrode 44. This increases the possibility of the occurrence of short circuiting between the transparent electrode 43 and the cathode 47. Accordingly, the insulation film 45 is requisite and thus increases the manufacturing cost. Further, the arrangement of the auxiliary electrode 44 at the both sides of the luminescent portion increases the area outside the luminescent portion on the glass substrate 42. In other words, the area of the surface of the glass substrate 42 that does not emit light is increased.
In the organic EL device 54 disclosed in Japanese Laid-Open Patent Publication No. 2003-123990, the cathode electrode terminal 53a is arranged on the opposite side of the anode electrode terminal 51a. As shown in FIGS. 7, 8A, and 8B, the auxiliary electrode 56 cannot be connected to the transparent anode layer 51 at the side farthest from the anode electrode 51a. Accordingly, the occurrence of brightness unevenness cannot be sufficiently suppressed. Referring to FIG. 8A, if the auxiliary electrode 56 were to be arranged along three sides of the transparent anode 51, this would increase the area outside the luminescent portion, or the area of the surface of the transparent substrate 50 that does not increase light.
Further, the insulation layer 55 is a requisite to the organic EL device 54 of FIG. 7. This increases the manufacturing cost. Furthermore, although not shown in the drawings, a seal cover or a passivation film is necessary to protect the organic EL layer 52 from oxygen or moisture in the ambient air.